feat(render): solid fill for PolyfaceMesh and PolygonMesh entities

PolyfaceMesh and PolygonMesh were wireframe-only. They now render with
the same flat-color solid fill as Face3D entities.

- Add fill_tris: Vec<[f32;3]> to TruckEntity and WireModel
- PolyfaceMesh: fan-triangulate each face into fill_tris
- PolygonMesh: triangulate each grid quad into fill_tris
- tessellate.rs Lines branch: apply world_offset to fill_tris
- Face3DGpu::from_wires: collect fill_tris from all wires in addition
  to Face3D key_vertices quads
- upload_face3d: pass all_wires so fill_tris are batched into one draw call

src/entities/arc.rs

@@ -87,6 +87,7 @@ fn to_truck(arc: &Arc) -> TruckEntity {
             snap_pts: vec![(cv, SnapHint::Center)],
             tangent_geoms: vec![tangent],
             key_vertices: vec![],
+            fill_tris: vec![],
         };
     }
 
@@ -101,6 +102,7 @@ fn to_truck(arc: &Arc) -> TruckEntity {
         snap_pts: vec![(cv, SnapHint::Center)],
         tangent_geoms: vec![tangent],
         key_vertices: vec![],
+        fill_tris: vec![],
     }
 }